Prepare the jars and lids: Wash all jars and lids thoroughly with soap and water and rinse well. Fill your canner with enough water to cover the jars by at least 1 inch and bring to a simmer. Using a pair of canning tongs, lower the jars in gently, tilting them to fill with the hot water. In a small saucepan, keep some water warm but not boiling; place the lids in the water. Have an additional kettle of water on to boil. Peel and core the tomatoes: Bring a large pot of water to the boil. Have a large bowl of ice water at the ready. Gently lower the tomatoes into the boiling water and blanch them for 60 seconds. Remove with a strainer and transfer them to the ice water. Once cool, slit the skins; they should peel off easily. Remove the cores with a small paring knife. You will need to do this in batches. Discard skins and cores. Prepare the sauce: Coarsely chop the tomatoes and add them to a large stockpot. Place stockpot over medium-high heat, crushing and stirring the tomatoes to keep from burning. Continue until all the tomatoes are added and crushed. Bring the tomatoes to a boil, then reduce heat and keep at a low boil. Reduce to the desired thickness, by a third for a thin sauce, or by half for a thick sauce. Fill and close the jars: Using canning tongs, remove the jars from the canner, carefully pouring the water back into the canner. Set next to the sauce. Turn the heat under the canner to high. Add two tablespoons of bottled lemon juice and one teaspoon of salt to each jar; add a teaspoon of sugar to offset the added acidity if desired. Use a ladle to pour the sauce into the jars through a canning funnel, leaving 1/2-inch headspace at the top. Run a clean chopstick around the inside of the jar to dislodge any trapped air. Wipe the rims of the jars with a damp paper towel. Place the lids on, and screw on the rings until just finger-tight. Seal the jars: Using canning tongs, gently transfer the jars to the canner, taking care to keep them vertical. When all the jars are in the canner, there should be at least 1 inch water covering them; if you need more, add water from the kettle until the jars are sufficiently covered. Bring the water to a full rolling boil, and process for 40 minutes. Remove and cool: Using canning tongs, gently remove the jars from the canner and transfer them to a kitchen towel or cooling rack, again keeping them vertical. Do not set hot jars directly on to cool counter surfaces. Leave to cool, undisturbed, for at least 12 hours. If any of the jars do not seal when cool, reprocess using the method above, or refrigerate and use immediately. Label and store: Add a label to the lid or side of your jar, noting the date it was canned. Remove the rings and store jars in a cool, dark place for up to one year. Refrigerate after opening.